Full 20m, 15m and 10m Band Coverage (with Feed Point Mounted ATU) Under 8 Feet Tall! No Radials!

Force 12’s Sigma Series Verticals are contest and DXpedition proven winners! The vertical dipole design means high efficiency even over poor ground. A vertical dipole will generally have a lower radiation angle and higher gain than a ground plane vertical mounted at the same height and achieves this with no ground radials. The “T-Bars” at the top and bottom of the Sigma define the compact but highly efficient design.

The SIGMA GT3 is a 2014 update of our venerable Sigma GT5. The ‘GT’ stands for ‘Garden Trellis’ as it is strong enough to hang a lightweight, non-conductive flower pot – or two – from the T-bars for backyard (or apartment deck) stealth operation. We have beefed up the vertical section to 1.500″ OD aircraft grade aluminum to support a remote autotuner (ATU) right at the feed point and done away with the old switched inductor system for improved bandwidth, match and power handling on 20m, 15m and 10m.

Here are the real world test results of an Elecraft KX3 feeding a MFJ-993BRT Intellituner ATU mounted at the SIGMA GT3 feed point, the antenna base at 4 feet above ground and fed through 50 feet of Buryflex (LMR-400 type) coax with the VSWR measured at the radio:

Frequency VSWR

14.025 1.5:1

14.195 1.4:1

14.345 1.4:1

21.025 1.2:1

21.295 1.1:1

21.445 1.2:1

28.025 1.2:1

28.495 1.3:1

Your performance may vary somewhat depending on your ground, mounting height and other environmental factors. The SIGMA GT3 is usable on 17m, 12m and 6m with reduced performance.

The 4 foot mounting height (measured at the base insulator) works well however the SIGMA GT3 will also work well mounted lower or higher including rooftop mounting. Higher mounting will lower the radiation angle.

The feedpoint is the most efficient place to locate an ATU – every foot of coax between your antenna and your ATU adds loss to the system when high VSWR is present on the feedline – and we offer a popular MFJ ATU which we have tested to work well with Sigma verticals and for which we have designed a rugged feed point mount. The MFJ ATU circuitry is powered by DC power sent down the coax so no separate control lines are required. Alternatively, you can supply your own remote – or desktop – ATU to use with your SIGMA GT3.

This is nothing like some of the flimsy verticals out there in the ham market – it’s a (little) tank! All of Force 12’s Sigma verticals are heavy duty and this model is HD top to bottom with 1.500 inch OD diameter aircraft grade aluminum tubing for the vertical section with sold fiberglass insulating section at the feed and 0.750 diameter inch T-bars. This rugged antenna is completely self-supporting, no guy wires needed. As with all Force 12 antennas, all hardware is stainless steel including the ATU mounting hardware.

The SIGMA GT3 HD is 7 feet 8.5 inches tall not counting the included 4 foot long 1.500 inch OD base which extends below a 2 inch wide solid fiberglass base insulator. The 58 inch wide T-bars are used for capacitive loading and are the reason we can make this antenna so short while maintaining a surprisingly level of performance. Every vertical antenna should be used with a feedline choke and we recommend the choke in the case of the Sigma ATU models be mounted as close to possible at the ATU input (assumes the ATU is mounted at the Sigma feedpoint). The feedline should be run perpendicular to the antenna at a 45 degree or greater angle to minimize coupling.

As the SIGMA GT3 is likely to be used in stealth operation in close proximity to living quarters we don’t recommend high power operation with this antenna although it could easily handle full legal power plus a bunch. Always be aware of FCC RF exposure regulations when installing and operating your amateur radio station.

When you order an MFJ Intellituner with a Force 12 Sigma Series vertical, Flagpole Antenna or InnovAntennas 33V Vertical Ground Plane we include the heavy duty feed point mounting bracket free (all stainless steel hardware).